Hi 14rvisaria, welcome to Thunkable! 
Be sure to check out our posts about How to ask Great Questions v2.0, our Community Guidelines, and our Docs as you get started.
While we’re glad you’re here, this is going to be a fairly complicated and difficult app to make, especially if you do not have previous coding experience.
We have a sample module for creating a user profile that you could use for this. In the blocks for setting the user’s profile, you could add a field for their birthday. Setting up a notification, you could use an alert or push notification to trigger on each user’s birthday.
For the chat portion, this is much more advanced but if you were going to start, you’d want to know how to use cloning for the UI so that when messages are sent, you can show the messages. For sending and storing messages between uses, the most feasible way to do this through a Firebase Realtime Database. This is where you could assign variables to each user and also store the messages each user would send and receive.
Again, this is a complicated and difficult task for even the most experienced Thunker.